1. Field of the Invention
The present invention relates to an optical disk used for recording/reproducing information and a method for producing the same.
2. Description of the Related Art
In the field of optical disks, there is a demand for high-density recording of information. As a method for realizing such high-density recording, a domain wall displacement detection (DWDD) system has been proposed.
In an optical disk of the DWDD system, it is required to weaken magnetic coupling between adjacent recording tracks. Therefore, in the case of producing an optical disk of the DWDD system, initialization for weakening magnetic coupling between adjacent recording tracks is conducted before recording an information signal. Such an initialization method has been reported in the past (see JP 6(1994)-290496 A and JP10(1998)-340493 A).
FIG. 10 shows an example of a structure and an initialization method of a conventional optical disk. As shown in FIG. 10, a conventional optical disk 1 includes a substrate 2, and a first dielectric layer 3, a recording layer 4, a second dielectric layer 5, and a protective coating layer 6 successively stacked on the substrate 2. On the surface of the substrate 2 on the side of the recording layer 4, grooves 2a are formed. A portion called a land is formed between two grooves 2a adjacent in the radial direction, and this portion becomes a recording track. The groove 2a has a width of, for example, 0.2 μm, and the land has a width of, for example, 1.4 μm. The recording layer 4 includes at least three magnetic layers for reproducing information by the DWDD system.
Next, an initialization method of the optical disk 1 will be described. According to the initialization method of the optical disk 1, laser light 7 (laser power=10 mW, λ=780 nm, NA of an objective lens 8=0.5, diameter of a light spot=about 800 nm) for annealing is radiated along the grooves 2a, thereby eliminating magnetic coupling of the recording layer 4 on the grooves 2a. During the initialization process, the relative moving speed of the light spot of the laser light 7 is, for example, 2 m/sec.
However, according to the initialization method as described above, portions other than the grooves 2a also are irradiated with a light spot. Therefore, an effective recording track becomes narrow, which decreases a signal level. This requires a light spot radiated to the recording layer 4 to be small. The first dielectric layer 3 is optimized in accordance with a wavelength of laser light for recording/reproducing, so that it is difficult to decrease the size of a light spot of the laser light 7 for annealing. Furthermore, for the same reason, it is difficult to enhance an absorption efficiency of the laser light 7 for annealing in the recording layer 4, so that initialization at a high linear velocity cannot be conducted, resulting in a long initialization time.